The present invention relates to an AC-DC converter in particular of the type comprising a switching element operated with less power loss and improved AC-DC conversion efficiency under a lower voltage of electric power supplied from an AC power source to the AC-DC converter.
A prior art AC-DC converter comprises a rectifying circuit with input terminals connected to an AC power source; a choke coil and a switching transistor connected in series to the rectifying circuit; a diode connected between the choke coil and one of output terminals; an output capacitor connected between the output terminals; dividing resisters connected in parallel to the output capacitor for presenting an output indicating the output voltage level; a power source for producing a reference voltage; an error amplifier for comparing the reference voltage and the output derived from the dividing resisters to produce an output related to a difference between the reference and divided voltages; and a PWM (Pulse Width Modulation) circuit for receiving the output from the error amplifier to generate drive pulses to a gate of the switching transistor. When the PWM circuit generates drive pulses, the switching transistor is turned ON and OFF alternately. During ON period of the switching transistor, electric current flows from the rectifier circuit through the choke coil and the switching transistor to charge the capacitor and store electric energy in the choke coil. During OFF period of the switching transistor, the choke coil discharges the stored electric energy to produce from the output terminals an output voltage raised by the output from the choke coil superimposed on the input voltage charged in the capacitor. When the output voltage is on a lower level, the error amplifier produces a lower level output to generate drive pulses of longer time width to the switching transistor, thereby turning the transistor ON for a longer period of time. Adversely, when the output voltage is on a higher level, the error amplifier produces a higher level output to generate drive pulses of shorter time width to the switching transistor, thereby turning the transistor ON for a shorter period of time,
Prior art AC-DC converters are required to keep the output voltage in a substantially constant and elevated level regardless of an input voltage level to the choke coil, and therefore they are driven from high to low step-up ratio of the DC output voltage to the AC input voltage. Specifically, when the input voltage changes between low and high level, they are operated to boost the output voltage to the elevated constant level between high and low step-up ratio in a wide range. When the AC-DC converters are driven to boost the input voltage with high step-up ratio, an excess current flows through the switching transistor, thereby resulting in decrease in AC-DC conversion efficiency and considerable power loss.
An object of the present invention is to provide an AC-DC converter which can prevent an excessive amount of electric current passing through a switching element for longer service duration of the switching element.
Another object of the present invention is to provide an AC-DC converter which indicates less power loss in a switching element with improved AC-DC conversion efficiency under supply of lower voltage electric power.
The AC-DC converter according to the present invention is provided with a booster circuit (9) which comprises a switching element (6) and a reactor (4) connected in series to the switching element (6) so that ON and OFF operation of the switching element (6) accumulates electric energy in the reactor (4) and discharges the electric energy from the reactor (4) to generate a DC voltage (VOUT) above an AC voltage (VIN) supplied from an AC power source (1). The converter also is equipped with a control circuit (10) which comprises potential means (11) for providing a reference voltage (VR); means (30) for comparing the reference voltage (VR) and DC output voltage (VOUT) and generating drive signals (VG) to turn the switching element (6) ON and OFF in response to the difference between the reference voltage (VR) and DC output voltage (VOUT); a voltage detector (18) for measuring an input voltage (VIN) applied on an input terminal of the reactor (4); and a voltage retainer (22) for maintaining the output voltage (VOUT) above the input voltage (VIN) measured in the voltage detector (18) by a substantially constant voltage.
Comparing the reference voltage (VR) and DC output voltage (VOUT), the control circuit (10) generates drive pulses (VG) for turning the switching element (6) ON or OFF to shorten or lengthen ON period of the switching element (6) when the output voltage (VOUT) is higher or lower respectively to reduce or increase the output voltage (VOUT). Also, as the voltage retainer (22) keeps the output voltage (VOUT) above the input voltage (VIN) measured in the voltage detector (18) by a substantially constant voltage (VP+VR or VZ), the booster circuit (9) simply raises the input voltage (VIN) by the substantially constant voltage (VP+VRor VZ). This means that the step-up ratio (VOUT/VIN) is smaller than that of a prior art converter for boosting the input voltage (VIN) up to a constant high DC output voltage regardless of a level of the input voltage (VIN). Accordingly, the converter according to the present invention may change the output voltage (VOUT) in a linear and ramp characteristic, whereas the prior art converter produces the output voltage of a fixed level with the changing step-up ratio (VOUT/VIN) in a wide range. The small step-up ratio (VOUT/VIN) causes less change in ON period of the switching element (6) and less amount of electric current (IDS) through the switching element (6) with reduction of power loss in the switching element (6) and improved AC-DC conversion efficiency.